The present invention relates to an image pickup apparatus. More particularly, the present invention relates to a compact and low-cost image pickup apparatus that is capable of photographing high-quality images comparable to silver halide photographs and that enables the depth of field to be controlled.
There are digital cameras for business use, e.g. printing, which are capable of obtaining images comparable in quality to photographs. These digital cameras have a large number of pixels, which exceeds 2 million pixels. However, the pixel pitch of the image pickup device is as large as about 7.5 to 10 μmm or more. Accordingly, the size of the image pickup device is large, i.e. more than 1 inch. Therefore, there is a limit to the number of devices obtained from one wafer. Consequently, the cost cannot be reduced in excess of a certain limit.
The most effective way of reducing the manufacturing cost of image pickup devices is to reduce the device size so that an increased number of devices are obtained from one wafer. However, it is necessary in order to reduce the device size to reduce the number of pixels or the pixel pitch.
Many of image pickup apparatus that have been commercially manufactured as relatively low-cost apparatus use image pickup devices of ⅓ to ⅔ inch size and with about 1 to 2 million pixels. With this order of numbers of pixels, when the image is enlarged to 8×10″ size (228 mm×186 mm) or more, it is impossible to obtain an image comparable in quality to photographs, as will be described later in detail.
The conventional apparatus use a lens system having satisfactory optical performance up to the Nyquist frequency, which is determined by the pixel pitch of the image pickup device. Thus, the conventional apparatus are intended to provide an image realizing the maximum resolution given by the number of pixels of the image pickup device with high efficiency.
In addition, when the apparatus need to be equipped with an optical low-pass filter in order to suppress the occurrence of a moire image and false color, the conventional practice is to adopt an arrangement taking into consideration the degradation of performance due to the low-pass filter. That is, the lens system is arranged to ensure sufficiently high optical performance to compensate for the degradation due to the low-pass filter, thereby allowing the optical low-pass filter to degrade the image-forming performance.
According to the above-described conventional method of approach, as the pixel pitch reduces, the Nyquist frequency becomes correspondingly high. Consequently, the optical performance required for the lens system becomes extremely high. In particular, if it is intended to improve the performance in the intermediate spatial frequency region, where the performance is degraded by the optical low-pass filter, the optical performance required for the lens system becomes almost the same as that of an ideal lens [see Japanese Patent Application Unexamined Publication (KOKAI) No. 10-148754].
In general-purpose image pickup apparatus, on the other hand, an aperture stop is provided in the lens system to enable appropriate photography to be carried out under a great variety of conditions. The aperture stop controls the amount of light reaching the image pickup device to obtain a correctly exposure-controlled image. If exposure is not correctly controlled, the resulting image contains many noises or becomes inferior in the detail reproduction in shadow areas.
Furthermore, if the diameter of a passing light beam is reduced by the aperture stop, the light beam is subjected to diffraction action strongly, and the point spread function by amplitude of the lens system expands. That is, so-called “diffraction unsharpness” occurs. Methods have been devised to prevent the occurrence of diffraction unsharpness [see Japanese Patent Application Unexamined Publication (KOKAI) Nos. 6-153066 and 11-8803]. According to one method, an ND filter is inserted to adjust the light quantity. According to another method, the exposure time is controlled with an electronic shutter to obtain correct exposure.
However, the effects produced by stopping down the aperture include the function of adjusting the depth of field as well as the function of adjusting the light quantity. Therefore, with the method in which the light quantity is adjusted by using an ND filter or the like, it is impossible to obtain the effect for the depth of field.
Diffraction unsharpness is a phenomenon that occurs in the optical system alone, totally independently of the pixel pitch of the image pickup device. Therefore, when the degree of unsharpness is the same, the smaller the pixel pitch, the greater the influence of diffraction unsharpness. Consequently, the image quality degrades correspondingly.
The degradation of the image quality due to diffraction unsharpness is a serious problem to the conventional idea of providing an image realizing the maximum resolution that the image pickup device possesses with high efficiency, as stated above. In particular, as the pixel pitch decreases, the effect of the image quality degradation due to diffraction unsharpness becomes stronger. Therefore, it is desirable that the aperture should not be stopped down in excess of a certain limit, or the pixel pitch should not be reduced in excess of a certain limit.
There has been provided a technique whereby even an image obtained at an aperture number at which diffraction unsharpness occurs can be made to appear sharp by subjecting the image to edge enhancement processing [see Japanese Patent Application Unexamined Publication (KOKAI) No. 5-347730]. Most of the image pickup apparatus that are commercially manufactured at present for consumer use employ the edge enhancement processing and provide images that appear sharp. Electrically, the edge enhancement processing can be applied almost unlimitedly. However, if an image is excessively subjected to the enhancement processing, when the image is enlarged, the image quality degrades extremely. Accordingly, it is desirable from the viewpoint of achieving high-quality images that the level of edge enhancement should be kept as low as possible. For this reason, the conventional image pickup apparatus for consumer use have not yet attained image quality comparable to that of silver halide photographs.
Thus, with the existing commercial products and prior art, when images are enlarged to 8×10″ size or larger, it is impossible to obtain images comparable in quality to silver halide photographs. In addition, according to the conventional method of approach, the pixel pitch cannot be reduced in excess of a certain limit. Therefore, it is difficult to achieve a favorably compact apparatus. Alternatively, the aperture number cannot be increased, and thus the depth of field cannot be controlled. Therefore, it is impossible to attain a compact and low-cost image pickup apparatus capable of obtaining high-quality images comparable to silver halide photographs.